I'd say you probably want the bestiary next based on what you have. I suggest your local gaming store for stuff if you can afford that, they are valuable to the gaming community as a whole and often don't have a high profit margin. That said it can get expensive so you can always use the PRD for things such as ultimate equipment if you con't want to buy the whole book.
http://paizo.com/pathfinderRPG/prd/
Seeking some advice...
1) The Pathfinder product list can be kinda intimidating. I have the Beginner Box, The Core Rule Book, and we're playing the Hell's Rebels Adventure set. What guide book do you suggest I buy next? I can't decide if I want to buy the Player's Guide or the Gamemastery guide. I've found the equipment in the core rule book to be very basic so I've considered buying the ultimate equipment guide as well. I've also considered the Pathfinder Campaign Setting: Cheliax since that's where our campaign is set. Any suggestions?
2) I can see how this would be a very expensive hobby very quickly. Do you guys have any suggestions for places on-line where you could possibly find used Pathfinder equipment? Even E-Bay is relatively expensive.
1) Advanced Player's Guide is a good one.
2) Get PDF versions regularly. If you're just using a feat or two here or there, use one of the online SRDs.